Yeah, I played some last night. I saw two ways to open the trap door: shooting the rollunder when the trapdoor light is lit, and the left side (behind the pop bumber) rollover switch.
FWIW, here's what the convoluted rulesheet says on the topic:
"LEFT RETURN ROLLOVER: Scores 500 points, adds Bonus and opens trap door"
"TRAP DOOR ROLLUNDER: Score 500 points and adds Bonus / Opens trap door when lit"
"ROLL DOWN TARGET: Score 5,000 points / Lights trap door when lit."
Unfortunately, I'm not sure where the "ROLL DOWN TARGET" is, nor how to light it... as a result, I can't really figure out how to light TRAP DOOR.